1. Field of the Invention
The present invention relates to a Direct Current (DC) offset correcting unit of a direct conversion receiver. More particularly, the present invention relates to a device and method for solving problems of the prior art in which channel information as well as a DC offset is removed when removing the DC offset using a feedback signal of a baseband amplifier.
2. Description of the Related Art
Each of most wireless communication transceivers selects and uses an analog heterodyne receiving method of down-converting a signal received therein into a baseband or low Intermediate Frequency (IF) through several mixers and IF terminals. However, there are problems in that a conventional transmission and reception structure using a plurality of analog components is complicated in its circuit and is difficult to be integrated as one chip. Therefore, there is a problem in that a transceiver using the transmission and reception structure is increased in volume.
In order to solve these problems, there is a direct conversion receiver composed of a Radio Frequency Integrated Circuit (RFIC). This structure composed of the RFIC will come into general use in a transceiver of a miniaturized and low-power system in the future. Because this direct conversion receiver performs only one frequency up and down conversion using one mixer, various filters, amplifiers, and peripheral circuits which must be used whenever one mixer is used may be reduced. Therefore, a system is simplified by minimizing an RF parts. This results in one-chip possibility of being increased. Finally, there is an advantage in that a unit cost may be reduced.
However, the direct conversion receiver generates many problems in its function. For example, a role burden to the mixer as well as an oscillation problem, a selectivity problem, etc. is increased. Particularly, the direction conversion receiver leads to a DC offset problem by leakage in a frequency of a local oscillator and a fatal problem such as noises generated near a DC band, etc.
In general, a DC offset may be easily removed by a DC-blocking capacitor. However, there is a problem in that it is required to provide a capacitor of a huge size to remove the DC offset in a baseband frequency band.
For this reason, there is a method of removing the DC offset by forming feedback by connecting a Low Pass Filter (LPF) or an integrator to a block which removes the DC offset.
As described above, because a High Pass Filter (HPF) function is generated in a closed-loop circuit forming the feedback, there is a principle in which the DC offset is removed by the HPF function. That is, if feedback is formed by connecting an LPF having a characteristic of β(s) to an amplifier having a frequency characteristic of A(s), a frequency characteristic Avf(s) of the entire closed-loop is approximated as a smaller value of 1/β(s) and A(s) in the Bode's diagram on a feedback characteristic and a HPF characteristic is implemented. Therefore, the DC offset is removed by the filtering by the HPF characteristic.
The DC offset may be removed by the above-described method. However, because not only the DC offset but also channel information in a low-frequency band is removed by the HPF characteristic, there is a problem in that an Error-Vector-Magnitude (EVM) or Signal-to-Noise Ratio (SNR) of a receiving terminal is degraded.
In order to solve the above-described problem, the loss of the EVM may be reduced by reducing a cutoff frequency of the direct conversion receiver to less than 1 to 10 KHz. In this case, because time of when the closed-loop is settled is lengthened, when a generated DC offset is changed, according to a channel environment, there is a problem in that to remove the DC offset keeps up with speed at which the channel environment is changed.
Therefore, it is required to provide a device and method for removing the DC offset in the direct conversion receiver to solve the above-described problems.